1. Field of the Invention
This invention relates to a camera shutter, and more particularly to a camera shutter the shutter blades of which move back and forth between two points of position under the influence of an electromagnetic force.
2. Description of the Prior Art
As is well known in the art, the shutter blades are designed to move back and forth between two points of position, as for instance those in the focal plane shutter. When the shutter release is actuated, the leading shutter blade runs in one direction. After a certain preset interval of time the trailing shutter blade runs in the same direction. In setting the focal plane shutter the leading and trailing shutter blades are made to return to the original position without admitting the light rays to the film. As for the slide type shutter, when the shutter release is actuated, two shutter blades each having an exposure aperture are brought to the aperture-opening position in which the exposure apertures of the shutter blades are laid on each other, and after a certain preset interval of time the shutter blades are brought to the aperture-closing position in which the apertures of the shutter blades are offset from each other.
The mechanical drive has been hitherto used to drive the shutter blades. This requires a relatively complicated structure to interlock and drive two shutter blades in the coordinating relation. Also, a reciprocating mechanism must be provided for each of the two shutter blades, and accordingly the inner structure of the camera is complicated, and hence the production cost is increased.
In an attempt to overcome these defects, it has been proposed that a plunger of an electromagnetic driving unit is connected to the shutter blade for opening and closing the shutter under the influence of the electromagnetic force. Specifically, when the electromagnetic driving unit is energized with electricity, the plunger is moved under the influence of the electromagnetic force, storing a resilient force in a spring. When the electromagnetic driving unit is deenergized, the plunger is returned to the original position under the influence of the resilient force, which is released from the spring. Partly because of the relatively heavy weight of the plunger and partly because of the counter force applied to the plunger by the resilient spring, the shutter blades cannot run at an increased speed. Still disadvantageously, it is difficult that two plungers associated with two shutter blades are made so as to run at the same speed. Because the resilient forces of the springs used cannot be exactly the same, and the electromotive forces to drive the plungers cannot be exactly the same, either.
If the two shutter blades travel at different speeds in the slide shutter, the size of the exposure aperture formed with the shutter blades is not stabilized, thus failing to accurately control the exposure.